Seismic exploration for oil and gas commonly involves towing a marine seismic streamer cable behind a towing vessel. Acoustic signals are set off in the water, and return signals as seismic data are acquired by acoustic sensors in the cable. A seismic acquisition system of the telemetry type includes electronic acquisition units (AUs) between and joining cable sections. The AUs acquire the seismic data and telemeter it to a central recorder. Each AU generally has an outbound command path and an inbound data path.
In systems deployed in typical field applications, the cables are often subject to stress and damage due to handling, cuts, scrapes, damages by animal life, pinching, and crushing because of their length and deployment.
It is therefore desirable to have redundant telemetry elements between AUs to increase the likelihood that a system will be intact and operational after being deployed into the water behind the towing vessel.
Many systems use optical fiber for the telemetry path. With an optical fiber, the most common redundant system has two optical transmitters firing into two fiber paths and two independent receivers. AU design is sensitive to both cost, size, and electrical power consumption. Since optical receivers are the largest and most power hungry portions of the AU telemetry system, it is desirable to minimize the number of additional optical receivers.
A fiber fails by either a complete loss of signal due to a break, or severe attenuation of the telemetry signal resulting in a signal strength that is below the sensitivity of the receiver. Unlike electrical paths, optical paths do not get noisy, have crosstalk, or get "shorted".
The problem of system malfunction due to conductor or fiber damage has existed as long as seismic systems, although redundancy of system components became practical in the 1970's when telemetry-based seismic systems were first introduced. Truly redundant fibers and conductors are expensive and found in very few systems because of the cost and complexity of duplicating and allocating high-bandwidth. In other words, duplicating high bandwidth circuits and transmission paths is expensive, and using lower bandwidth paths requires more of such telemetry paths for the same data throughput. Generally, two transmitters, two data paths, and two receivers plus voting/selection circuitry is required for redundancy. The material, physical space, cost, and power consumption are disadvantages of including any such redundancy in a telemetry system.
There are no known redundant fiber seismic acquisition systems in the market today. Currently, cables with failed optical fibers require replacement of the damaged cable sections at a great cost in repair time, effort, and lost productivity.
The present invention provides a low cost system of redundant optical fiber data path backup, using minimal space, power, and cost. Although two transmitters, and two data paths are required, only one receiver, a costly and power-hungry part of an optical telemetry system, is required. Coupling of the data-carrying laser light is accomplished using a device known in the optical fiber art as a coupler, or by means described below in greater detail, thereby coupling multiple fibers into a single connector. Typical optical loss penalty is 3 dB (50%) using a coupler, versus about 0.4 dB (10%) for the present invention.